The present invention relates to a stiction-free microstructure releasing method for fabricating a device of a microelectro-mechanical system (MEMS), and in particular, to a method of fabricating a MEMS device by means of surface micromachining without leaving any stiction or residues by etching silicon oxide of a sacrificial layer, which is an intermediate layer between a substrate and a microstructure, rather than by etching silicon oxide of a semiconductor device.
When using silicon oxide as a sacrificial layer and using a silicon as a microstructure, an HF solution is generally used for wet etching to remove the silicon oxide. A cleansing solution such as de-ionized (DI) water, methanol or isopropyl alcohol is used to rinse and remove the remaining HF solution.
However, using this method poses a problem of leaving the cleansing solution in a gap of a micron unit between the microstructure and the substrate in the drying process for evaporating the cleansing solution. A capillary force is generated by a surface tension due to the residue. If the capillary force becomes greater than the retrieving force, the microstructure tentatively sticks on the substrate. Such a tentative sticking is changed into a permanent surface sticking due to the van der Waals force, electrostatic force or hydrogen bridging. Such a phenomenon is referred to as a stiction.
Further, a reaction between the HF gas and methanol or isopropyl alcohol results in H2O due to a chemical reaction. At that time, the silicon oxide undergoing a removal process meets evaporating water drops, thereby leaving diverse kinds of residues that are not evaporated on the substrate due to condensation. FIG. 1 shows a photograph of such stiction and generation of residues.
FIG. 5 shows an SEM photograph of a vapor etching process. Stiction occurs and residues remain on the incompletely etched silicon oxide, which is a sacrificial layer, due to the scum, side wall polymerization and decorations of HF remaining on the etched silicon oxide.
To avoid such stiction and residues, researches have been conducted in diverse aspects. The methods presented at the initial stage of the research were reducing the contacting surface of the silicon by roughing and widening the surface area thereof, or undergoing an NH4F process for the silicon so as to be hydrophobic. However, these methods failed to release the microstructure on a revivable basis. The methods suggested recent days were a supercritical CO2 drying method (the path Axe2x86x92B in FIG. 2) utilizing the characteristics of a phase transfer to a supercritical fluid by lowering the pressure after moving the cleansing solution to a supercritical region and converting the cleansing solution to a fluid, which is an intermediate state between gas and liquid, and a sublimation method of directly solidifying the cleansing solution by using sublime materials such as t-butyl alcohol, p-DCB (dicholorobenzene), etc. (the path Axe2x86x92C in FIG. 2) without undergoing a liquid state. However, these methods also pose a problem of failing to completely removing the water, failing to release the silicon microstructure on a revivable basis, or being complicated in its process while being very difficult in handling the testing pieces, thereby being uneconomical and inappropriate for massive production.
Accordingly, a demand has been raised to develop a method for effectively and revivably releasing the microstructure with a simple drying process of not resulting in stiction or generation of residues.
It is, therefore, an object of the present invention to provide a new method of releasing a microstructure on a completely separate basis without resulting in stiction or leaving residues by using anhydrous HF gas and alcohol vapor as well as by using a thermodynamic nature of water against temperature and pressure for etching silicon oxide of a sacrificial layer.
To achieve the above and other objects, there is provided a microstructure releasing method for fabricating a MEMS device according to the present invention comprising the steps of: supplying an alcohol vapor bubbled with anhydrous HF; maintaining a temperature of the supplying device and the moving path to be higher than a boiling point of the alcohol; vapor etching by controlling the temperature and pressure to be in a vapor region of a phase equilibrium diagram; and removing silicon oxide of a sacrificial layer on a lower portion of a microstructure.
The pressure for etching is preferably 25-75 torr with the temperature ranged 25-80xc2x0 C. A process that may be performed before the etching is a vapor etching of a part of the silicon oxide. The silicon oxide of a sacrificial layer is preferably one or more components selected from the group consisting of TEOS, LTD, PSG, BPSG and a thermal silicon oxide, while the alcohol is preferably one or more components selected from the group consisting of methanol, isopropyl alcohol and ethanol. The MEMS device may be of a laminated structure or a single crystal structure.
To achieve the above and other objects, there is also provided a method for removing silicon oxide of a sacrificial layer for a microstructure, characterized by removing the silicon oxide of a sacrificial layer by means of a vapor etching with anhydrous HF and alcohol and by controlling the temperature and pressure of an etch chamber to be in a vapor region of a phase equilibrium diagram of water.
According to the method for removing the silicon oxide of a sacrificial layer for the microstructure of a MEMS device, the pressure in the etch chamber is preferably 25-75 torr with the temperature ranged 25-80xc2x0 C. The pressure below 25 torr or the temperature higher than 80xc2x0 C. delays or disables the etching. On the other hand, the pressure above 75 torr of the temperature lower than 25xc2x0 C. undesirably accelerates the condensation process.